[FEATURE] Add externalMetadata option for glTF and GLB

[xeolabs]

Add an externalMetadata option for use when converting .gltf and .glb source files that are accompanied by metadata .json files.

This will cause the metadata JSON files to not be embedded within the XKT output files, and instead be output separately.

{
    "sourceConfigs": {
        "las": {
            "center": false,
            "transform": [
                1.0, 0.0, 0.0, 0.0,
                0.0, 0.0, -1.0, 0.0,
                0.0, 1.0, 0.0, 0.0,
                0.0, 0.0, 0.0, 1.0
            ],
            "colorDepth": "auto",
            "fp64": true,
            "skip": 1,
            "minTileSize": 200
        },
        "laz": {
            "center": false,
            "transform": [
                1.0, 0.0, 0.0, 0.0,
                0.0, 0.0, -1.0, 0.0,
                0.0, 1.0, 0.0, 0.0,
                0.0, 0.0, 0.0, 1.0
            ],
            "colorDepth": "auto",
            "fp64": true,
            "skip": 1,
            "minTileSize": 200
        },
        "ifc": {
            "excludeTypes": [],
            "minTileSize": 200
        },
        "gltf": {
            "reuseGeometries": true,
            "includeTextures": true,
            "includeNormals": false,
            "excludeTypes": [],
            "minTileSize": 200,
            "externalMetadata": true
        },
        "glb": {
            "reuseGeometries": true,
            "includeTextures": true,
            "includeNormals": false,
            "excludeTypes": [],
            "minTileSize": 200,
            "externalMetadata": true // <------- 
        },
        "json": {
            "center": false,
            "transform": [
                1.0, 0.0, 0.0, 0.0,
                0.0, 0.0, -1.0, 0.0,
                0.0, 1.0, 0.0, 0.0,
                0.0, 0.0, 0.0, 1.0
            ],
            "minTileSize": 200
        }
    }
}